Introduction

Background to ticks as parasites of birds

 

Ticks have been implicated as being the vectors of many diseases although at present there is little documented research on the association of ticks and the death of wild birds or even the association of ticks causing disease and no death (Pomykal: 1981). Schilling et al (1981) described an incidence of tick infestation (Ixodes arboricola) in 7 of 40 broods of peregrine falcons in Germany, all infested broods being from cave sites, with a high percentage of infested birds dying.

Tick associated deaths have however been recorded in captive birds (Fig 1), and it is indeed well recognised that aviary birds from the UK can suffer acute death following tick infestation (Forbes and Fisher: 2000). Hard ticks (Ixodidae) and soft ticks (Argasidae) have been implicated in causing associated death, but with only I. frontalis, I. arboricola and I. ricinus as of yet being identified with all clinical cases of acute deaths.

     
 
 
 

Fig 1 A adult female hard tick engorging on a falcon (taken from http://www.intfalconer.com/back_issues/5article.html)

Perhaps the most significant article, when discussing acute deaths is by Forbes and Simpson (1993). They recount the experiences at their own veterinary practice of five cases in the two previous years of birds succumbing to acute effects of ticks (Ixodes ricinus). The signs of tick attachment were similar in all cases, however death was observed in only three of the five cases. The first two cases were aviary birds, a parakeet and a peregrine. Both were closely observed frequently and on the previous day both had been eating and appeared fit and well. The following day both birds were found dead in their aviaries; each showed subcutaneous haemorrhage and oedema of the head. With a most pronounced reaction in the 2 to 3 cm area around the site of tick attachment. Post mortems revealed no significant findings and it was concluded that death had arisen as an acute response to a tick bite. The other three cases occurred in falconer’s birds and were thus handled frequently and examined thoroughly. The first bird a gyr falcon also presented with gross oedema and haemorrhage around the point of tick attachment. The bird fist seemed to respond to treatment but then represented with circulatory failure. Necropsy revealed a significant haemorrhaging ulcer in the proventriculus. The last two birds had similar signs to the gyr falcon, with a grossly swollen and oedematous proventriculus, as viewed endoscopically, but both made uneventful recoveries. The cases in which birds have not died indicates that either some species of bird are not affected by the ticks or that only certain ticks have the ability to cause death, this suggesting that the mechanics of the tick biting the host is not a cause of death.

The mechanism of how ticks can cause an acute death in their hosts is unclear. It could either be a disease transmitted by the ticks acting as vectors, a hypersensitivity reaction to the ticks saliva components or it could be a toxicosis. As of yet there has not been one clear mechanism identified with cases. With regards to the latter mechanism Forbes and Simpson (1993) recite Rothschild and Clay (1952) who state that ‘the saliva that is poured into the host’s flesh can be highly toxic, and the eggs contain poisonous substances which may prove fatal, whereas blindness may follow attachment of a tick near a bird’s eye’.

There have been a few others that have speculated on the cause of death.  Chastel et al (1981) describe death in a collared dove (Streptopelia decaocto) as being from a neurotoxin injected from a tick (I. frontalis) as it feeds, however their virological studies on the bird’s tissues and of the tick were negative. These findings were compatible with tick paralysis or a minor equivalent to this syndrome. Chastel et al (1991, 1999) go on to state that I. frontalis is responsible for severe pathological disorders leading to the death of some birds under peculiar conditions and that it can transmit fatal viruses.

Hillyard (1996) and Luttrell et al (1996) write about tick paralysis, which is capable of causing death in wild birds. Hillyard states that the toxic substances are neurotoxins which inhabit the neuro-muscular junctions and Luttrell; that tick paralysis is considered a ‘motor polyneuropathy characterised by a progressive, ascending flaccid motor paralysis …. which may result in rapid death’.

Knott (1993) recounts the post mortem examinations on two aviary birds that died suddenly both with a single tick (I. ricinus) attached to the top of the head. Both birds presented with subcutaneous haemorrage around the site of tick attachment but with no other symptoms and appeared to be in good condition with regards to everything else. Although a similar incidence was reported a short while afterwards in the same aviary of one of the deceased birds, Knott put the deaths down to shock and thought that it was unlikely that the deaths were due to tick-borne disease.

Previous studies (performed by former students of Southampton University) on acute deaths of captive birds has revealed that there is widespread distribution of the types of birds affected, the most frequently affected being from the family Psittacidae (the parrots) and that there is no correlation between the cases and their distribution with regards to a north-south divide in the UK. The epidemiology of tick associated death in wild birds has previously had little attention but has particular relevance to a global distribution as wild birds are recognised as a natural means of distributing ticks over long distances and thus they can be an important factor in the spread of tick-borne diseases. There has been many references for birds helping the spread of Lyme disease, for example (Olsen et al 1995, Ishiguro et al 2000, Slowik and Lane 2001).  Slowik and lane (2001) show that birds contribute to the spread of Lyme disease in three ways. First, migrating birds can facilitate the spread of tick vectors and the spirochete, Borrelia burgdorferi, causing Lyme disease. Second, some birds may serve as primary hosts of vector ticks and lastly birds may function as sources of spirochetal infection for uninfected vector ticks. The same may apply to the epidemiology of tick associated death, if it is a transmissible pathogen, as some birds not succumbing to the effects will be able to spread the disease. 

1.2 Aims of the project

After consultation with Neil Forbes FRCVS (Avian veterinarian at Lansdown veterinary hospital who has undertaken similar research) and Dr. Clive Bennett (project supervisor) it was decided that a project on tick associated death should be concentrated on epidemiology of the issue in wild birds. The year before a similar project was used to identify cases in aviary birds. The results observed will hopefully shed some light on this scant researched subject and be of benefit to rescue centres in making them more aware of the problem.

 

The main aims of this study will thus be to:

For all the aims it should be noted that data was collected for 2001 only.

 

Results

The following tables and figures are divided into four sections the first corresponding to the incidences of ticks on birds and those of illness and death. The second section refers to the type of bird more commonly dying as a result of tick attachment and the third section represents the response rates of each type of questionnaire. After every table the G-statistic value will be given in the format discussed in the method. The fourth section presents some findings that were insufficient to enable statistics to be used.

 

For each test performed if there is an non significant G(adj)-statistic value (P>0.05), that is a statistical value that is below the critical value for the appropriate degrees of freedom, as assessed on a chi-squared table, then the null hypothesis H0 has to be accepted. This infers that there is no difference in the proportions of class between the different samples. Furthermore this means that the class type is independent of the type of sample. If a test produces a significant value above the critical value then the H0 has to be rejected. Thus the conclusion can be drawn that there is a statistically significant difference in the proportions of class between the samples and that the class is dependent on the sample.

 

For each G-statistic value that is significant the P represents the probability that the difference observed is down to chance alone. If P<0.05, this represents a 5% probability that the statistical difference seen between variables is down to chance alone. P< 0.01 and P< 0.001 are equivalent to a 1% and 0.1% chance respectively. Obviously the less probability the results are down to chance alone the stronger certainty of the difference in samples although any G-statistic value that is above the lowest critical value (P= 0.05) is designated as significant.

 

3.1 Incidences of ticks on birds

The tables in this section refer to the numbers of observed birds for each class and sample and were used to calculate the G-test statistic. From each type of questionnaire sent the returns could be categorised into areas, country then region within country, so that distribution of incidences could be compared. Out of the possible incidences available from the observed frequencies only ‘birds with ticks against total birds’, ‘birds affected by tick attachment against birds with ticks’ and ‘birds dying from tick attachment against birds with ticks’ are statistically assessed.

Incidences of birds between the UK, USA and other countries

The following three contingency tables refer to, as their legends explain, the number of birds observed in each class for three different samples: the UK, USA and all the other countries the questionnaire was sent to. As the tables are 3 x 2 there is no need to apply William’s correction, as it would be negligible.

Table 3.1 Numbers of birds with tick(s) attached and numbers of birds without tick(s) attached for the UK, USA and all other countries combined.

Country
Birds with tick(s) attached Birds without tick(s) attached Total
UK

170

18765

18935

USA

199

32808

33007

OTHER

260

16463

16723

Total

629

68036

68665

G = 102.777, P<0.001 (2df)

In conclusion the incidence of birds with ticks attached is strongly statistically different between the UK, USA and all other countries combined. This G value is so large mainly because of the other countries. Although all samples have small percentage incidences because the total number of birds is so high, the other sample has a much larger relative incidence (1.6%) than both the UK (0.9%) and USA (0.6%). The value of a G without this sample is much lower, G = 14.379, P<0.001 (1df), however it is still significant owing to the fact that just the UK and USA also show a significant difference in their incidence to one another.

 

Table 3.2 Numbers of birds affected from tick attachment and numbers of birds with tick(s) attached not affected for the UK, USA and all other countries combined.

Country

Birds affected from attachment Birds with tick(s) but not affected Total
UK

64

106

170

USA

58

141

199

OTHER

21

239

260

Total

143

486

629

G = 63.055, P<0.001 (2df)

The difference in incidence of birds affected by attachment is thus strongly significant between the UK, USA and all other countries combined with the USA having the highest incidence. Again the sample from the other countries seems to be contributing to the difference observed the most. The UK and USA have incidences of 37.6% and 29.2% respectively where as the other countries have an incidence of only 8.8%. Moreover if the UK and USA are compared without the other category the G value is non-significant, G = 2.976, NS (1df).

 

Table 3.3 Numbers of birds dying from tick attachment and numbers of birds with tick(s) attached not dying (including those affected and not affected) for the UK, USA and all other countries combined.

Country

Birds dying from attachment Birds with tick(s) but not dying Total
UK

7

163

170

USA

13

186

199

OTHER

6

254

260

Total

26

603

629

G = 5.061, NS (2df)

In conclusion the difference in incidence of birds affected by attachment is not significant between the UK, USA and all other countries combined. Further tests for the number of birds dying against either total birds or birds affected by attachment also gave non significant results (G= 0.196, and G= 4.567, respectively) thus strengthening the NS result observed here.

 

For tables 3.1 and 3.2 the proportion of birds observed is highly dependent on the sample region. For table 3.3 the proportion of birds is independent on the region where the frequencies were observed. This is due to similar incidences for each sample region ranging from 2.4% - 6.5%.

 

Incidences of birds between regions in the UK

The following five contingency tables refer to, as their legends explain, the number of birds observed in each class for two different samples: North and South for the UK and East and West for the USA. As the tables are only 2 x 2 William’s correction has been applied thus Gadj is equivalent to G in these cases. For the counties or states contained in each region of each country refer to appendix 8 (this also shows the incidences that the statistics are based on here for each region).

 

Table 3.4 Numbers of birds with tick(s) attached and numbers of birds without tick(s) attached for the North and South regions of the UK.

Region

Birds with tick(s) attached Birds without tick(s) attached Total
North

21

8925

8946

South

149

9794

9943

Total

170

18719

18889

Gadj = 95.970, P<0.001 (1df)

From this value it can be stated that the incidence of birds with ticks attached is strongly statistically different between the north and south regions of the UK. This G value is so large because the incidence for the south is much greater than the north at 1.5% and 0.2% respectively.

 

Table 3.5 Numbers of birds affected from tick attachment and numbers of birds with tick(s) attached not affected for the North and South regions of the UK.

Region

Birds affected from attachment Birds with tick(s) but not affected Total
North

5

16

21

South

59

90

149

Total

64

106

170

Gadj = 2.019, NS (1df)

This G value is not significant because the incidence between the two regions is similar relative to the sample size. A test for the number of birds dying against the number of birds with ticks could not be performed between the north and south because of insufficient data that meant the expected results would have been below the minimum of 5 which is required for a reliable test (Fowler, cohen and Jarvis 1998).

 

Incidences of birds between regions in the USA

Table 3.6 Numbers of birds with tick(s) attached and numbers of birds without tick(s) attached for the East and West regions of the USA.

Region

Birds with tick(s) attached Birds without tick(s) attached Total
East

83

17267

17350

West

116

15541

15657

Total

199

32808

33007

Gadj = 9.444, P<0.01 (1df)

This G value means that the samples have a relatively large significant difference for the incidence of birds with tick(s) attached between the two countries which cannot be put down to chance alone.

 

Table 3.7 Numbers of birds affected from tick attachment and numbers of birds without tick(s) attached for the East and West regions of the USA.

Region

Birds affected from attachment Birds with tick(s) but not affected Total
East

5

78

83

West

53

63

116

Total

58

141

199

Gadj = 42.019, P<0.001 (1df)

In conclusion the incidence of birds affected by tick attachment is strongly statistically different between the east and west regions of the USA. This G value is so large because the incidence for the west is much greater than the east at 45.7% and 6% respectively.

 

Table 3.8 Numbers of birds dying from tick attachment and numbers of birds with tick(s) attached not dying (including those affected and not affected) for the East and West regions of the USA.

Region

Birds dying from attachment Birds with tick(s) but not dying Total
East

1

82

83

West

12

104

116

Total

13

186

199

Gadj = 7.770, P<0.01 (1df)

This G value represents a relatively large significant difference, between the incidence of birds dying from tick(s) attached, between samples. It may appear on first glance that the this value should be higher as the number of birds dying in the west region is 84.60% higher than the east however, in relation to the sample size this difference is reduced to 9.15% between the two regions, which is still high enough to give a statistical difference but not as high as one would assume if sample size was not taken into account. 

 

3.2 Type of bird more commonly dying from tick attachment

The G-test was applied here to see if there was a statistical difference in the incidence of birds dying between the order Passeriformes (passerines) and all other orders of birds combined (non-passerines). For table 3.9 the total number of passerines and non-passerines was calculated by asking the centres the percentage of each type of bird made up the total. Thus the average proportion for all centres that reported a death could be calculated. This was then used to multiply the total number of birds with ticks attached at the centres reporting deaths to gain the total numbers of birds.

 

Table 3.9 Numbers of birds dying from tick attachment and numbers of birds with tick(s) attached not dying (including those affected and not affected) for two large bird type categories: passerines and non-passerines (values only from ‘Internet’ version of questionnaire.)

Bird type

Birds dying from attachment Birds with tick(s) but not dying Total
Passerine

6

52

58

Non-passerine

13

40

53

Total

19

92

111

Gadj = 3.879, P<0.05 (1df)

This value is marginally significant and would benefit from collecting a larger sample size. The incidences of birds dying from attachment as a proportion of the total number of birds with a tick attached are 10.35% and 24.53% for passerines and non-passerines respectively.

 

3.3 Response rate to different types of questionnaire

The response rates are measured as a percentage of the returned questionnaires from the total questionnaires sent to valid addresses (Fig 3.1). The number of invalid addresses for the mailed, attachment and internet questionnaire sent were 20, 40 and 7 respectively. Table 3.10 gives the values used to calculate the G-test to see if there is a significant difference in the number of questionnaires returned in respect to the totals sent between the different types of questionnaire.

Fig 3.1 Total valid addresses for the three types of questionnaire sent and the number of returns for each type giving a response rate for each.


 

 

Table 3.10 Numbers of questionnaires returned and numbers of questionnaires not returned for the Mailed, Attachment and Internet types of questionnaire sent.

 

Questionnaire type Returned Not returned Total

Mailed

41

370

411

Attachment

46

234

280

Internet

62

221

283

Total

149

825

974

 

G = 18.925, P<0.001 (2df)

The explanation for such an outcome is that there is a significant difference in the response rates between the mailed, attachment and internet questionnaires with the internet questionnaire producing the largest response rate and the mailed the lowest. If just the attachment and internet types of questionnaire are tested against one another in a 2x2 contingency table the outcome is not significant: Gadj  2.721= , NS (1df).

3.4 Statements without statistics

 

For many of the questions not enough data was collected to produce any valid observations.  However, from the table in appendix 9 a few statements can be made regarding the birds that died from tick attachment, although this data was insufficient for statistical analysis.

1.      The most frequently observed family of birds were the Columbidae

2.      The most frequently seen symptom was swelling around the tick insertion point

3.      The most frequently recorded amount of ticks on a bird that died equalled one

4.      The most frequently seen position of ticks on birds was on the head region

 

Discussion

Incidence and their distribution                                                                                                                                                 

These have produced mixed probabilities, some being highly significant and others ranging to non-significant, meaning that some incidences (tables 3.1, 3.2, 3.4, 3.6, 3.7, 3.8) are dependent on the region of the sample and others are not (tables 3.3 and 3.5). The reasons for such a difference of tick on birds are hard to pin down to any one explanation alone. One can only assume that the countries (table 3.4 and West USA, table 3.6) with a other category, table 3.1) and regions (South UK, higher incidence have more suitable habitats for birds and ticks to interact sufficiently and the local and regional climate may also cause differences. Hilton (1991) reports of experiences as a bird bander in that, mild winters seem to lead to greater infestations of ticks. Osacar-Jimenez et al (1998) report of a significant difference in the area of where birds were caught and their levels of infestation, with a higher prevalence being recorded in mountain valleys than any other biotopes. Furness (unpublished article) refers to areas of long grass having higher numbers of ticks and puts this down to a more humid environment promoting enhanced survival of ticks. However, empirical evidence of caught wild birds has produced varied infestation rates for different regions that are inconclusive to the to the results obtained here (see below for details, Stafford et al (1995) and Hubalek et al (1996) etc.).

Perhaps the biggest factor contributing to differences is the local area surrounding the rescue centres. Clearly urban centres will have a lower risk area than countryside centres. For example, a region containing many centres may have less suitable habitats but individual centres may be situated in local areas with different tick abundance. Also, Bennett (1995) notes when referring to birds and Lyme disease, that host potential depends on the availability of the host. Applied to this data it can be inferred that the incidence of ticks infesting birds is equally effected by the number of ticks and the number of birds.

Explaining the results obtained for tables 3.2, 3.3 and 3.5 is hard to define. For table 3.2 it can only be assumed that regional and climatic differences between countries has an affect on the incidence of ticks causing illness in the birds, this is opposite to the findings in table 3.5 in which incidence of birds affected is independent to its region. Referring to the non-significant result in table 3.3 the incidence of birds dying in each country is unrelated to its geographical position. However, Neil Forbes (personal communication) stated that tick paralysis is more an American and Australian problem, thus the proportion of acute deaths making up the incidences of birds dying may be different.

In the USA the west region has a significantly higher incidence of birds affected and birds dying than the east of the country (tables 3.7 and 3.8). This is however mainly due to the results of one of the centres in the west region having reported 50 birds affected by ticks and 10 dying (appendix 7 and 9) suggesting that this local region is severe. However, it may also suggest that some centres are more proficient at spotting ticks on birds and then associating the possible cause of death with the tick.

Other reasons for differences in incidences

Stafford et al (1995) report that out of the individual ticks’ parasitising the birds they caught, 82.7% of the ticks were larvae. Hilton (1991) reports that ‘some common tick species’ immatures are only the size of the period at the end of a sentence, while adults are slightly smaller than a capital letter "O”’. Thus it is reasonable to assume that many of the ticks parasitising the birds were too small to be noticed. Another reason why ticks would have gone unnoticed is due to little or no search effort for the ticks at the rescue centres. Out of the all of the replies it was noted that 19 of the centres admitted that they did not specifically look for ticks. The following two comments are extracts from two centres replying from the USA.

‘We have never scrutinised the bodies for ticks and therefore have not encountered ticks on the admitted raptors.  They could be there but we missed them.’

‘In order to find ticks on a bird, a thorough exam of its body under its feathers would have to be done. We receive many birds and we do not have the kind of time that would be required for these exams.’

This figure is probably less than the actual number of centres not searching for ticks as some centres may have thought such a comment would show incompetence, thus this does cast some doubt on to the relevance of the results obtained. However, this was unavoidable in the type of research because the findings were totally dependent on respondent’s views. Still one suspects that the data compiled here is an underestimate of the true incidence of birds with ticks attached and of those succumbing to illness or death. 

This is further supported by the findings of many field experiments in which wild birds were caught to assess the population infestation rate. From the USA: Stafford et al (1995) report 15.2% of total birds caught in Connecticut having ticks attached; Manweiler et al (1990) observed 49% of the 138 birds captured in California; Durden et al (1997) from South Carolina caught a total of 259 birds of which 45 presented ticks (17%); Kinsey et al (2000) reports of a 14% infestation rate of 423 birds caught in Georgia and Alabama. Outside the USA: Hubalek et al (1996) report that 29% of 411 forest birds caught, in the Czech Republic, were being infested and Nosek and Balat (1982) report of their findings that on 323 birds caught in southern Sweden, 77 ticks were collected from seven species of bird. However, Nosek and Balat don’t record the number of birds with ticks on, thus all 77 ticks could have been collected from a minimum of 7 birds. However, even if this were the case the percentage of birds with ticks attached would still be 2.2% minimum. Furness (unpublished data) refers to having ‘removed nearly 5,000 ticks from roughly 10,000 birds’ in Scotland, although no incidence of infestation can be extracted from this data it represents and average of 0.5 ticks for every bird!

These figures on the whole are a drastic increase of what was found in this study (0.9% incidence of infestation), especially as the birds visiting rescue centres are compromised in some sense. Thus one would expect more ticks on these individuals. Pomykal (1981) presumes that infestation is more likely if birds are injured in such a way that it means that they are exposed to tick-infested habitats. For example, if a bird was unable to fly it would inevitable be at a higher risk to infestation.

However, some of the reports only represent a few months thus this could be at a peak time for tick activity, where as in this report the whole of 2001 was included thus every season was covered.  Also, Manweiler et al (1990) point out that because their method of capture with Japanese mist nets (which was the method applied by most of the others referenced) only captured birds if they flew down to below 2 metres it suggests that the larger samples of species caught would be those that spend more of their time on the ground or in low level vegetation. Thus these percentages give an infestation mainly for ground birds and it is likely that these percentages are higher when compared to the results obtained here because data collected from this research represents a wider distribution of species. Thus it could be argued that from the incidence of ticks reported here one would expect it to be less than in these field experiments.

Incidence of birds dying and the type of bird

Referring to the significant difference between the incidence of passerines dying from tick attachment being lower than that of non-passerines (table 3.9), it is well appreciated that ground birds are more commonly affected than birds that spend less time on the ground (Hilton 1991, Slowik and Lane 2001). Bennett (1995) cites the work of others in finding ticks on 149 ground-inhabiting birds. These types of bird will be more prone to picking up ticks than birds that spend less time on the ground. This idea was also highlighted by a couple of comments made by respondents.

It is not uncommon to see birds that feed on seeds to have ticks attached around the head region.’

‘I would think more terrestrial type birds to pick up ticks commonly, but not raptors who spend a greater proportion of their time in the air’.

From the results obtained in this research it would seem to contradict this, as passerines are more commonly associated as being ground birds than non-passerines. But if the orders of birds are examined it can be seen that Columbiformes (the pigeons and doves), which are non-passerine ground feeders make up 12 of the 26 cases of birds dying. Thus contributing to the difference more than any other group.

Acute deaths in cases of birds dying

It is impossible to tell which cases of birds reported as dying represent an acute death as the agent that causes death has not been identified yet, thus no clear species of tick vector can be identified either. There were no reported cases of species of ticks that have previously been associated with acute deaths in captive birds (I. ricinus, I. frontalis and I. arboricola). Of the five individuals of tick identified as causing death here (appendix 9) they were of two species, I. angustus and I. holocyclus. Ixodes angustus may well be a likely candidate for causing an acute death as the case history of the two birds that it caused death in are similar to those cases of acute death reported in captive birds. This species of tick has also been identified as being a competent vector of Lyme disease (Peavey et al 2000), however analyses on these particular ticks were negative for the presence of Borrelia burgdorferi, the spirochete causing Lyme disease. Thus the tick must have caused the death of the bird by some other means, possibly by acute death.

Ixodes holocyclus is known as the Australian paralysis tick and is the most common tick to cause paralysis in Australia. The three cases of birds dying in Australia were all found with a I. holocyclus tick found attached to them and when found they were still alive but unable to move. This is a classic symptom of tick paralysis as described by Grattan smith et al (1997). Thus as the correct symptom and associated tick for tick paralysis are present on these three cases it is highly likely that they were not examples of an acute death.  The case with the Cormorant (also appendix 9) may also have been tick paralysis rather than acute death as the symptoms given describe ‘respiratory problems’ which is another symptom of tick paralysis (Luttrell et al 1996).

However, in the 26 presumptive cases of tick paralysis put forward by Luttrell out of those that necropsy was performed on, the birds ranged from fair to good physical condition with gross lesions limited to subcutaneous haemorrhage and oedema at the tick attachment sites. This is in conjunction with the findings of Forbes and Simpson (1993) (see below for more details) of birds succumbing to acute deaths following tick attachment. Thus at least some of the cases reported by Luttrell may have in fact been the result of an acute type of death. Moreover, Chastel (1981) compares the parasitism of a Collared dove (Streptopelia decaocto) with an unidentified neurological syndrome (possibly the same as that causing acute death) as being compatible with tick paralysis or a minor equivalent of this syndrome.

The cases with the two Harris hawk nestlings (appendix 9) are similar to the findings of Schilling et al (1981). In Schilling et al’s cases numerous ticks were present on chicks, up to 320 per chick. Seventy four percent of infested ticks died, which with such high levels of infestation is hardly surprising. This could account for the death of these two birds rather than an acute type of death.

For the rest of the cases of birds dying no particular conclusions could be formed because of the lack of data. However, the statements in the results section referring to appendix 9 were consistent with findings detailed from Forbes and Simpson (1993) and cases highlighted in the similar project completed last year on captive birds dying in the UK. Forbes and Simpson (1993) report of ticks most frequently attaching to birds head region where they cannot remove the ticks by preening. In most cases of acute deaths reported by Forbes and Simpson, there has been swelling around the insertion point of a single tick. Lutrell et al (1995), Hilton (1991) and Bennett (1995) report of that ground feeders are the most frequently found birds with ticks attached. As Columbidae are examples of ground feeders it is unsurprising to notice that they have the highest number of birds affected.

Final thoughts on tick associated death

In reflection there may be a lower reported incidence of acute deaths because by the time of death, the localised swelling around the tick may have subsided, and the tick may even have become detached (Neil Forbes, personnel communication). Forbes and Simpson (1993) when reporting of their own experience of acute deaths in birds describe a case with a gyr falcon that was examined very carefully and daily by its owner, in particular the head region, but the following day it was presented at their veterinary hospital with gross oedema and subcutaneous haemorrhage in a 3cm area around the point of insertion of a tick (I. ricinus). This presents a situation in which the owner could have missed the tick. Forbes and simpson go on to recount their findings of necropsy on the bird and other like wise affected and state that swelling may not remain for long after death and if the skin of the head is not removed the signs of haemorrhage and oedema maybe missed. Bennett (1995) reports the findings of others that birds have a shorter duration of infection than rodents. Thus ticks could be attaching and unattaching themselves before the birds are coming into the rescue centres.

The difficulty highlighted by many of the aforementioned researchers is that conformation of cases of disease in wild birds is hard to ascertain to be tick related because the condition is usually not noticed until the bird is found dead or severely affected by the tick attachment in which time the tick may have moved off the host.

Newton (1993) referring to nestling raptors, also points out that disease may be underestimated because freshly young dead maybe eaten by their nest mates or parents, thus not remaining long as evidence. This may also be true of adults succumbing to tick associated death in that their remains will almost certainly be scavenged before too long.

Many of the respondents said that ticks were less of a problem on birds than other ectoparasites such as mites, flies and lice. In particular louse flies/flat flies (Hippoboscidae) seemed to be the most frequently commented on ectoparasite affecting wild birds. An article on the Internet by Forbes (http://www.intfalconer.com/back_issues/5article.html) states thatflat flies are frequently found on wild birds and cause irritation but also may transmit blood-born infections. Whilst a host may be adapted to its own parasites, if these are transferred to an abnormal host, they may cause a massively increased disease…. experiences of clients raptors after killing corvids, are subject to hippoboscidae subsequently transferring to the raptor, leading to sudden death due to blood parasite infestation within 7 days.’ This could cause confusion with acute deaths caused by ticks.

Indeed ‘confusion’ seems to be the right word associated with acute deaths. Neil Forbes (personal communication), indicated that after some examinations of birds dying, which had been diagnosed with severe haemorrhages as the cause of death, it was believed that the bird must have died from a severe collision, by flying into something for example. However, an attached tick causing an acute death can as already mentioned cause similar haemorrhages. Thus, in some of these circumstances, even with experienced veterinarians, the connection with an attached tick has previously been overlooked and it has only been when the problem was brought to their attention that they have realised a possible connection.

 

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