EXOTIC ANIMAL DISEASE NEWSLETTER
April 2008 - Volume 2, Issue 1
Horse orbivirusesAfrican horse sickness (AHS)
AHS is an orbivirus that can be spread by the same vectors as those responsible for the dissemination of bluetongue and epizootic haemorrhagic disease, a disease of deer and cattle.
AHS has recently occurred in Portugal (1989), Spain (1987-90), Zimbabwe (2004), Botswana (2005), Namibia (2006), Eritrea (2006), Lesotho (2007), Nigeria (2007), Senegal (2007) and South Africa (2007). Whilst Serotype 9 is most widespread in endemic areas, the outbreaks in Botswana, Nigeria and Senegal were due to AHS serotype 2. This is important when determining which vaccine to use.
Transmission is via Culicoides sp. vectors. Incubation period can be 2-14 days. Clinically there is fever and malaise followed by swelling of face, neck, shoulders and brisket (cardiac form) or dyspnoea, coughing and frothy discharge from nostrils (pneumonic form). Mortality rates as high as 95% have been recorded in horses. Donkeys, mules, zebra, dogs, camels and elephants may also be infected. AHS is a notifiable disease in Australia.
13 horses died in Western Cape province in South Africa’s AHS surveillance zone in February/March 2007. Initially the cause was attributed to AHS, but further investigation revealed that equine encephalosis virus serotype 1
(EEV-1) was the cause. EEV-1 was also detected in Culicoides sp. in the affected area. It was noted that there were an unusually large number of Culicoides sp. caught in the area. Equine encephalosis is caused by an orbivirus unrelated to AHS. It usually causes a subacute or mild disease with fluctuating fever, listlessness and inappetence. It may result in a peracute illness with signs of nervous or cardiac involvement. Facial swelling, ataxia, depression, convulsions and respiratory distress have been described and these symptoms are the result of severe endothelial damage. First identified in 1967, it has only been reported to date in Africa. Horses and zebras are the only hosts known to be affected. Equine encephalosis is a notifiable disease in Australia.
The recent outbreaks of BTV-8 in Europe has highlighted new concerns about the potential spread of orbiviruses like AHS out of their traditional endemic areas due to the extension of the range of the vector.
Porcine reproductive and respiratory syndrome (PRRS)
PRRS was first detected in USA in 1987 and spread to the major pig producing areas around the world. Australia and New Zealand remain free. PRRS causes significant economic loss due to reproductive failure in sows and respiratory distress in piglets and growing pigs. Pigs are the only known susceptible species. In 2006/07 reports emerged from China and Vietnam of a disease causing high fever and mortality in pigs across a range of age groups. Initially a combination of PRRS, classical swine fever and circovirus was suspected. Investigations showed the cause to be a new variant of PRRS virus, which emerged in 2006, and differed from the European and USA strains.
In Vietnam mortality rates of up to 24% were attributed to concomitant or secondary infections. In China many adult pigs and pregnant sows died. In 2006 more than 2 million pigs were affected and 400,000 died. Commercial and backyard farms were affected. The disease reemerged in 2007 again with high losses, so high that the price of pork in China increased.
In October 2007 the Chinese authorities reported that PRRS was under control following the implementation of a compulsory vaccination program involving more than 100 million pigs and using a new vaccine that matched the circulating strain.
The most likely path of entry of PRRS to Australia would be by the feeding of uncooked infected meat (swill feeding) to pigs.
A bacterial disease for a change. Glanders still occurs. During 2007 it was reported in Philippines, India, Iran, Norway and Russia; in 2006 in Eritrea and Brazil; in 2005 in Mongolia and in 2004 in the United Arab Emirates in post-import isolation. All cases involved the movement of latent cases in horses. Control was dependant on the slaughter and disposal of infected and in-contact animals. The mallein test is not be very effective for detecting early cases.
Caused by Burkholderia mallei (Pseudomonas mallei ), glanders is an important zoonosis, as untreated human cases are usually fatal. It occurs in horses, mules and donkeys and is often chronic or latent. Dogs and cats may also be infected. Infection is usually by ingestion or contamination of skin abrasions. It causes nodules and ulcerations in the upper respiratory tract and lungs. In the acute form there is fever, coughing and thick nasal discharge with ulceration of nasal mucosa. Submaxillary lymph nodes are swollen and painful. In the chronic form there is general malaise and unthriftiness with chronic nasal discharge and possible skin lesions (farcy), mainly on the legs.
Dogs and ticks
Mention ticks and dogs and on the east coast of Australia, we think of tick paralysis. But in the USA lyme disease (Borrelia burgdorferi) may be higher on the list. A recent study by IDEXX in USA has shown lyme disease, anaplasmosis (Anaplasma phagocytophilum) and ehrlichiosis (Ehrlichia canis) are present in dogs in all states.
Lyme disease was prevalent in the northeastern states (up to 18% of dogs testing positive), while anaplasmosis was more common in the midwest and ehrlichiosis in the southeast.
Rocky mountain spotted fever (Rickettsia rickettsii) is regarded as the most severe tick borne illness in USA. The vectors include the American dog tick (Dermacentor variabilis), the Rocky Mountain wood tick (Dermacentor andersoni) and the brown dog tick (Rhipicephalus sanguineus).
Clinical signs in dogs include joint stiffness and seizures. Not all infected animals show clinical signs.
In humans it causes fever, muscle pain and headache followed by a rash. Humans at greatest risk are those exposed to dogs and ticks. Cases most frequently occur from April to September each year when ticks are most prevalent. No country can consider themselves remote from the risk of introduction of diseases.
The recent outbreak of equine influenza in Australia is a good example of how the globalization of trade in animals and their biological products increase the risk.
Microbiologists at Oregon State University have discovered a new species of myxozoan parasite that has been found for the first time in a warm blooded animal. Myxozoa are a group of spore forming microscopic parasites that live in marine and fresh water habitats. They are found in fish with life cycle stages developing in invertebrates. Some are host specific but others have a broad range of hosts. Whirling disease in trout and salmon is an myxosporean parasite.
The new parasite called Myxidium anatidum was discovered in a duck from a zoo that died of unknown causes. Nine isolated cases in free flying and captive birds have now been reported across the USA. The parasite appears to target the liver, with mature spores found in bile ducts. Investigations are proceeding.
Samples were collected from healthy cattle at Beatrice Hill east of Darwin between April and June 2007 as part of the National Arbovirus Monitoring Program (NAMP). Viruses were isolated at Berrimah Veterinary Laboratories and grouped as bluetongue but could not be allocated to any of the known Australian serotypes. Further testing at the Australian Animal Health Laboratory at Geelong showed the viruses to be of the Australia A genotype but not of an existing Australian genotype. In April 2008 the virus was identified as BTV-7—the first time this serotype had been detected in Australia.
BTV-7 has been reported sporadically from Africa and south east Asia. It has not been associated with disease in cattle. It is believed that the virus entered Australia in windborne vectors. Further investigations are currently underway.
New diseases do occur. This has been brought home by recent experiences both in Australia and overseas. Each time we examine an animal and the clinical signs are not typical of what we expect in that species in our area we need to think of exotic diseases in our list of differential diagnoses.
You may be looking at the first case.
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Further information on these diseases can be obtained from the AUSVETPLAN website