Terrestrial Animal Health Code
Terrestrial Animal Health Code |
Infection with Trypanosomabrucei, T. congolense, T. simiae and T. vivax
General provisions
This chapter adresses a disease complex caused by several protozoan parasites of the genus Trypanosoma, transmitted mainly cyclically by the genus Glossina (tsetse flies), but also mechanically by several biting flies (e.g. tabanids, Stomoxys spp.). The disease can be caused by many different trypanosomes and can affect various mammals such as horses, donkeys, camels, goats, sheep, pigs, dogs, cats and non-human primates. The disease has a significant socio-economic impact on animal production. Trypanosoma brucei gambiense and T. brucei rhodesiense can also affect humans and are responsible for a disease known as sleeping sickness or human African trypanosomosis, which is almost always fatal if untreated.
Infection with several trypanosome species in the same animal may occur although this may not always be detected using routine testing methods.
For the purposes of this chapter, “susceptible animals” means domestic and wild animals from the following families: bovidae, suidae, equidae, camelidae, canidae, felidae and non-human primates.
For the purposes of the Terrestrial Code, infection with Trypanosoma brucei, T. congolense, T. simiae and T. vivax is defined as an infection of susceptible animals with one or more salivarian trypanosomes of the subgenera Duttonella (only T. vivax), Nannomonas (only T. congolense and T. simiae) and Trypanozoon (T. brucei sspp. excluding T. evansi and T. equiperdum), hereafter referred to as “pathogenic agent”.
Infections of susceptible animals with T. evansi and T. equiperdum are covered by Chapter 8.X. and Chapter 12.3., respectively.
Other trypanosomes including T. uniforme, T. godfreyi and T. suis, which are rarely reported and of limited distribution and impact, do not play a significant role in the epidemiology of the disease; however, they should be considered in the surveillance system owing to their interference (hidden infection) with the diagnosis of infection with T. brucei, T. congolense, T. simiae and T. vivax.
The following defines the occurrence of infection with T. brucei, T. congolense, T. simiae and T. vivax:
the pathogenic agent has been observed in a sample from a susceptible animal; or
genetic material specific to the pathogenic agent has been detected in a sample from a susceptible animal showing clinical signs consistent with infection with T. brucei, T. congolense, T. simiae and T. vivax or which has an epidemiological link to a confirmed case; or
antibodies have been detected in a sample from a susceptible animal showing clinical signs consistent with infection with T. brucei, T. congolense, T. simiae and T. vivax or which has an epidemiological link to a confirmed case.
For the purposes of the Terrestrial Code, the incubation period of infection with T. brucei, T. congolense, T. simiae and T. vivax shall be 90 days.
Standards for diagnostic tests are described in the Terrestrial Manual.
Safe commodities
When authorising the import or transit of the following commodities from susceptible animals, Veterinary Authorities should not require conditions related to infection with T. brucei, T. congolense, T. simiae and T. vivax regardless of the status of the exporting country or zone:
pasteurised milk and pasteurised milk products;
hair, wool and fibre;
gelatine and collagen;
horns, hooves and claws;
meat from animals that have been slaughtered in a slaughterhouse/abattoir and have been subjected to ante- and post-mortem inspections with favourable results;
hides and skins (except raw);
semen collected and processed in accordance with Chapter 4.6.;
embryos.
Country or zone free from infection with T. brucei, T. congolense, T. simiae and T. vivax
A country or zone may be considered free from infection with T. brucei, T. congolense, T. simiae and T. vivax when:
the infection is notifiable in the entire country;
measures to prevent the introduction of the infection have been in place; in particular, the importations or movements of susceptible animals and other commodities into the country or zone have been carried out in accordance with this chapter and other relevant chapters of the Terrestrial Code;
and either:
the relevant provisions in point 2 of Article 1.4.6. have been complied with; or
for at least the past two years, there has been no case in the country or zone and
surveillance in accordance with Articles 8.19.7. to 8.19.10. has been in place in the entire country; or
the absence of competent vectors has been demonstrated by a surveillance programme in accordance with Chapter 1.5. and Article 8.19.9.
A country or zone free from infection with T. brucei, T. congolense, T. simiae and T. vivax adjacent to an infected country or zone should include a zone in which surveillance is conducted in accordance with Articles 8.19.7. to 8.19.10.
Compartment free from infection with T. brucei, T. congolense, T. simiae and T. vivax
The establishment and bilateral recognition of a compartment free from infection with T. brucei, T. congolense, T. simiae and T. vivax should follow the provisions laid down in this chapter and in Chapters 4.4. and 4.5.
Susceptible animals in the free compartment should be protected against the vectors by the application of an effective biosecurity management system.
Recovery of free status
Should a case of infection with T. brucei, T. congolense, T. simiae and T. vivax occur in a previously free country or zone, its status may be recovered after the following:
infected animals have been isolated and then immediately treated, slaughtered or killed and appropriately disposed of;
animals in contact with infected animals have been put immediately under protection from vector attacks and tested;
AND
for six consecutive months, either:
after the last case was slaughtered or killed, the animals in contact have undergone monthly repeated serological and agent detection tests with negative results in both tests; or
when treatment is applied to the infected animals, both treated and in contact animals have undergone monthly repeated serological and agent detection tests with negative results in both tests;
AND
surveillance in accordance with Articles 8.19.7. to 8.19.10. has been carried out with negative results;
appropriate biosecurity is in place, including vector control or protection from vector attacks in the affected area.
Otherwise, Article 8.19.3. applies.
Recommendations for importation of susceptible animals from countries, zones or compartments free from infection with T. brucei, T. congolense, T. simiae and T. vivax
Veterinary Authorities should require the presentation of an international veterinary certificate attesting that the animals:
showed no clinical signs of infection with T. brucei, T. congolense, T. simiae and T. vivax on the day of shipment;
were kept since birth in a free country, zone or compartment or were imported from a free country, zone or compartment;
did not transit through an infected zone during transportation to the place of shipment or were protected from vectors or any source of the pathogenic agent by the application of effective biosecurity during transportation to the place of shipment.
Introduction to surveillance
Articles 8.19.7. to 8.19.10. define the principles and provide guidance on surveillance for infection with T. brucei, T. congolense, T. simiae and T. vivax, complementary to Chapter 1.4. and to Chapter 1.5.
The purposes of surveillance could be the demonstration of the absence of infection, the early detection of cases, or the measurement and monitoring of the prevalence and distribution of the infection in a country, zone or compartment.
Vectors are an essential component of the epidemiology of the salivarian trypanosomes. Therefore, the surveillance system should include a vector surveillance component to detect the presence and estimate the abundance of tsetse flies. When appropriate, it should also allow estimation of the vector infection rate with the pathogenic agent. Vectorsurveillance may also assist with estimation of the abundance of mechanical vectors.
The impact and epidemiology of the pathogenic agent widely differs between different regions of the world and therefore, it is not appropriate to provide specific recommendations for all situations. Member Countries should provide scientific data explaining the epidemiology of the disease in the country or zone concerned and adapt the surveillance strategies for defining their status to the local conditions. There is considerable latitude available to Member Countries to justify their status at an acceptable level of confidence.
Although surveillance in wildlife presents challenges that may differ significantly from those in domestic animals, wildlife should be considered in the surveillance system because they can serve as reservoirs of infection and as indicators of risk to humans and domestic animals.
General conditions and methods for surveillance
A surveillance system in accordance with Chapter 1.4. should be under the responsibility of the Veterinary Authority. In particular, it should include:
a formal and ongoing system for detecting and investigating outbreaks of disease;
a procedure for the rapid diagnosis in the field or for the collection and transport of samples from suspected cases to a laboratory for diagnosis;
a system for recording, managing, reporting and analysing diagnostic and surveillance data.
The surveillance programme for the pathogenic agent should, at least:
in a free country or zone, have an early warning system which obliges animal owners and keepers and other stakeholders who have regular contact with susceptible animals, as well as veterinarians or veterinary paraprofessionals, to report promptly any suspicion of infection with T. brucei, T. congolense, T. simiae and T. vivax to the Veterinary Services.
An effective surveillance system will periodically identify suspected cases that require follow-up and investigation to confirm or exclude whether the cause of the condition is the pathogenic agent. The rate at which such suspected cases are likely to occur will differ between epidemiological situations and cannot therefore be reliably predicted. All suspected cases should be investigated immediately, and samples should be taken and submitted to a laboratory;
include random or targeted serological or parasitological surveys appropriate to the status of the country or zone.
Surveillance strategies
The target population should include domestic and wild susceptible animals of epidemiological significance within the country or zone. Active and passive surveillance for the pathogenic agent should be ongoing as epidemiologically appropriate. Surveillance should be composed of random or targeted approaches using parasitological, serological, clinical and entomological methods appropriate for the status of the country or zone.
In a free country or zone, it is appropriate to focus surveillance in an area adjacent to an infected country or zone, considering relevant ecological or geographical features likely to interrupt the transmission of the pathogenic agent.
A Member Country should justify the surveillance strategy chosen as being adequate to detect the presence of infection with T. brucei, T. congolense, T. simiae and T. vivax in accordance with Chapter 1.4. and Chapter 1.5., and with the prevailing epidemiological situation.
If a Member Country wishes to declare freedom from infection with T. brucei, T. congolense, T. simiae and T. vivax in a specific zone, the design of the surveillance strategy should be targeted to the susceptible population within the zone.
For random surveys, the sample size selected for testing should be large enough to detect evidence of infection if it were to occur at a predetermined minimum expected prevalence. The sample size and expected prevalence determine the level of confidence in the results of the survey. The Member Country should justify the choice of the minimum expected prevalence and confidence level based on the objectives of surveillance and the epidemiological situation, in accordance with Chapter 1.4. Irrespective of the survey approach selected, the sensitivity and specificity of the diagnostic tests employed are key factors in the design, sample size determination and interpretation of the results obtained. Ideally, the sensitivity and specificity of the tests used should be validated for the infection history and the different species in the target population.
Irrespective of the testing system employed, surveillance system design should anticipate the occurrence of false positive reactions. If the characteristics of the testing system are known, the rate at which these false positives are likely to occur can be calculated in advance. There should be an effective procedure for following up positive reactions to determine, with a high level of confidence, whether they are indicative of infection or not. This should involve both supplementary tests and follow-up investigation to collect diagnostic material from the original sampling unit as well as those which may be epidemiologically linked to it.
The principles involved in surveillance are technically well defined. The design of surveillance programmes to prove the absence of infection with T. brucei, T. congolense, T. simiae and T. vivax should be carefully followed to avoid producing results that are either insufficiently reliable to be accepted by international trading partners, or excessively costly and logistically complicated.
The results of random or targeted surveys are important in providing reliable evidence that no infection with T. brucei, T. congolense, T. simiae and T. vivax is present in a country or zone. It is, therefore, essential that the survey is thoroughly documented. It is critical to consider the movement history of the animals being sampled when interpreting the results.
An active programme of surveillance of susceptible populations to detect evidence of infection with T. brucei, T. congolense, T. simiae and T. vivax is essential to establish the animal health status of a country or zone.
Clinical surveillance
Clinical surveillance aims to detect clinical signs of infection with T. brucei, T. congolense, T. simiae and T. vivax in susceptible animals, particularly during a newly introduced infection. However, neither clinical nor post-mortem signs of infection with T. brucei, T. congolense, T. simiae and T. vivax origin are pathognomonic. Therefore, suspected cases of infection with T. brucei, T. congolense, T. simiae and T. vivax detected by clinical surveillance should always be confirmed by direct or indirect laboratory tests that confirm the presence of the pathogenic agent.
Parasitological surveillance
Parasitological surveillance can be conducted to:
Molecular techniques
Molecular techniques increase the sensitivity of the detection of active infections. They can also be applied to identify the parasite and to better characterise the genotype of circulating parasites in a country or zone.
Molecular techniques can be used to:
detect an active infection,
characterise the parasite at the species, subspecies, group and population level.
Serological surveillance
Serological testing of susceptible animals is one of the most effective methods for detecting exposure to the pathogenic agent. The host species tested should reflect the epidemiology of the disease. Management variables that may influence likelihood of infection, such as the use of insecticides or animal treatment, should be considered.
Owing to cross reactions with T. evansi, T. equiperdum, T. cruzi and Leishmania spp., the presence of these pathogenic agents should be considered when interpreting the results of the serological surveillance system.
Serological surveillance can be used to:
Positive test results can have different possible causes:
Sentinel animals
Sentinel surveillance may provide evidence of freedom from infection or provide data on prevalence and incidence as well as the distribution of disease or infection. Sentinel surveillance may consist of:
the identification and regular testing of one or more of sentinel animal units of known health or immune status in a specified geographical location to detect the occurrence of infection with T. brucei, T. congolense, T. simiae and T. vivax;
the investigation of clinical suspect cases targeting highly susceptible animals such as dogs, donkeys or horses.
Vector surveillance
This point should be read in conjunction with Chapter 1.5.
For the purposes of this chapter, vector surveillance aims at determining different levels of risk by identifying the presence and abundance of various vector species in an area or by demonstrating the absence of vectors. Demonstration of the absence of competent vectors may support the claim of freedom from infection with T. brucei, T. congolense, T. simiae and T. vivax.
The most effective way of gathering vector surveillance data should consider the biology and behavioural characteristics of the local vector species and include traps, fly rounds, sticky targets or other collection tools. The choice of the number and type of collecting tools to be used and the frequency of their use should be made by considering the size and ecological characteristics of the area to be surveyed.
When sentinel animals are used, vectorsurveillance should be conducted at the same locations.
Additional surveillance procedures for recovery of free status
In addition to the general conditions described in this chapter, a Member Country seeking recovery of country or zone free status, including a containment zone established in accordance with Article 4.4.7., should show evidence of an active surveillance programme to demonstrate absence of infection with T. brucei, T. congolense, T. simiae and T. vivax.
Populations under this surveillance programme should include:
establishments in the proximity of the outbreak;
establishments epidemiologically linked to the outbreak;
animals moved from or used to re-populate affected establishments.
nb: first adopted in 2021; most recent update adopted in 2024
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