Understanding Anaplasmosis–Tickborne Diseases in Uganda
Ticks are among the most important vectors of pathogens that cause some of the most deadly tickborne diseases (TBDs) in the livestock sector in East, Central and Southern Africa. In Uganda, the three most economically debilitating tick species affecting the country’s cattle herd are Amblyomma variegatum (bont-legged tick), R. appendiculatus (brown ear tick), and R. decoloratus (African blue tick). The latter causes Anaplasmosis in cattle, also known as Bovine Anaplasmosis, a tickborne disease whose prevalence in Uganda ranks second only to that of East Coast Fever. This article provides an in-depth overview of Anaplasmosis, its causative agent, regional distribution, clinical signs, as well as new control and preventative measures anticipated to help farmers safeguard their livestock herd, including a look at TicVac-U®, the new candidate vaccine for combating the deadly Anaplasmosis disease.
An overview of Anaplasmosis in Uganda
Besides East Coast Fever, another economically significant tick-borne disease in Uganda is Anaplasmosis, also known as Bovine Anaplasmosis (Gall sickness). It is transmitted by a rickettsial organism, Anaplasma marginale, parasites whose principal vector in Uganda is the R. decoloratus (African blue tick).
Distribution of Anaplasmosis
Since Anaplasmosis is transmitted by blue ticks, its distribution overlaps the distribution of its principal vector. In Uganda, the African blue tick is prevalent in the south-western, north-eastern and central cattle corridor (62%) as well as the north-western regions (82%), indicating extensive exposure of indigenous cattle to Anaplasmosis and a state of endemic stability.
Both the blue and brown ear tick species are prevalent in the same regions of Uganda, which can lead to concurrent transmission of East Coast Fever and Anaplasmosis in cattle since both ticks can be found feeding on the same herd. Anaplasmosis is, however, more common during wet seasons when hematophagous insects and mosquitoes are more abundant to facilitate mechanical transmission.
How does Anaplasma cause Anaplasmosis cattle disease?
Once cattle are infected with Anaplasma parasites, they penetrate the red blood cells and start to multiply, leading to a doubling of infected red blood cells every 24- 48 hours. The incubation period varies but usually falls between 4 and 10 weeks after tick bite transmission.
Once the red blood cells are infected, the animal's immune system recognises them as foreign to the body and are subsequently broken down and removed by the liver, spleen and lymph nodes. This is what leads to anaemia, a drastic condition that can result in cattle losing up to 70% of their red blood cells. This only becomes noticeable once the animal has lost more than 40-50% of its red blood cells.
Clinical signs
What are the clinical signs of Anaplasmosis in cattle? Some clinical signs and diagnostic symptoms of Anaplasmosis infection in cattle include high temperature (fever), progressive anaemia (blood loss), weight loss, rumen stasis, constipation and icterus.
In the advanced stages, the animal’s mucous membranes (gums, eyes and preputium) are first pale before turning into the more common yellow (icteric) due to jaundice. The yellow colour comes from the broken-down haemoglobin (red blood cell pigment) in the liver.
It is worth noting that calves younger than 6 months are more resistant to Anaplasmosis than adult cattle, which succumb to the disease with a very high mortality rate. Calves often develop a milder form of Anaplasmosis from which they recover not only fast but also become immune carriers of the infection.
In endemic regions that have achieved enzootic stability for Anaplasmosis, such as Kenya and South Africa, calves barely a few weeks old are systematically exposed to ticks so they get infected and develop immunity. The calves survive infection and become carriers, often without showing any symptoms of the disease.
Diagnosis and treatment
A presumptive diagnosis can be made on the clinical signs. However, the early stages of Anaplasmosis can easily be confused with Redwater (Babesiosis), another common tick-borne disease in Uganda that presents similar symptoms and is also transmitted by blue ticks.
A definitive diagnosis has to be made microscopically on a blood smear to ascertain infection. Carrier animals can be diagnosed with serology tests before they are introduced into the herd.
Once diagnosed, the best treatment for Anaplasmosis in cattle is tetracycline. The only challenge for livestock farmers is that the drugs are expensive, limiting their use in the field. Many rural areas in Uganda further lack adequate availability of vet services as well as drug supply, which constrains the prevention and control of TBDs.
There is also the issue that Anaplasmosis sometimes occurs at the same time as East Coast Fever. This can occur if blue ticks and brown ear ticks both feed on cattle, and both diseases are transmitted as a result. But more common is that an animal has developed ECF, causing a depression of its immune system, thus allowing an anaplasma parasitaemia infection, which may have been present in a carrier state to flare up as clinical Anaplasmosis. It may take only one week or so for this to happen.
However, any animal that has both diseases is likely to be very sick because, in addition to suffering from the symptoms of East Coast Fever, it will also be anaemic from Anaplasmosis. Unless suitable treatment is given promptly for both diseases, such an animal is likely to die very quickly because of severe symptoms.
Control and Prevention of Anaplasmosis
The control of Anaplasmosis (gallsickness) depends on 2 strategies, tick control and vaccination. Tick control in Uganda has been attempted primarily through spraying and dipping of cattle in chemical acaricides to control tick populations.
Unfortunately, this control method has failed and has simply led to an emerging situation of ticks becoming resistant, on top of acaricide contamination of meat, milk and the environment.
There is no existing vaccine for tickborne Anaplasmosis in Uganda. Unlike Babesiosis, blocking against the Anaplasma parasite isn’t possible. What has worked in other countries is strategically sustaining a stable disease situation to prevent large losses. In endemic regions, calves are, for instance, allowed exposure during the period when they are protected by maternal antibodies until they become subclinically infected and develop immunity.
In enzootic areas with adequate numbers of blue ticks, most adult cattle do become naturally immune to Anaplasmosis through repeated exposure, a situation where natural infection of livestock occurs and high levels of immunity are maintained, achieving epidemiological stability. This is the most economical and practical option for eradicating Anaplasmosis in endemic regions.
But this option is not always possible or practical, for one because susceptibility to Anaplasmosis often increases when there isn’t enough blue tick vectors to maintain natural infection and immunity or when non-immune adult cattle are introduced into enzootic areas.
Non-immune older cattle are thus vulnerable to Anaplasmosis once they are moved into an endemic area. This is also why the disease can be very severe in older cattle compared to calves.
In these situations or when vector or environmental conditions suggest an increased risk of Anaplasmosis, vaccination can be beneficial. It can offer farmers the ability to enhance or modulate the immune system, thus aiding in the control and prevention of Anaplasmosis in cattle.
The feasibility of using anti-tick vaccines to control and prevent other tick-borne diseases has already been demonstrated in many other countries.
In Uganda, clinical trials are currently underway involving the use of a candidate anti-tick vaccine to help cattle develop immunity against Anaplasmosis and other tick-borne diseases by increasing antibody levels to ensure immunity. The anti-tick vaccine is being developed by the Anti-Tick Vaccine Development Initiative, a presidential initiative aimed at developing and commercializing an anti-tick vaccine in Uganda, headed by its Principal Investigator, Dr. Margaret Saimo-Kahwa.
Below, we talked to Dr. Saimo-Kahwa on the various aspects of the anti-tick vaccine and its workings.
What was the reasoning behind an anti-tick vaccine?
“Due to the successful use of an anti-tick vaccine elsewhere in Latin America as a practical solution to controlling TBDs, it has led us to develop an anti-tick vaccine using local ticks, one that will be user-friendly and low-priced.”
How does the anti-tick vaccine work?
“The Anti-Tick Vaccine “TicVac-U®” is our brand. It has been tested, is immunogenic and works by eliciting antibodies against the tick tissues. The antibodies, when taken in by the ticks during the blood meal, destroy the ticks’ gut, leading to rupture and leakage of fluids into the tissues. This leads to the death of the ticks before laying eggs.”
“Due to malabsorption of nutrients in the gut leads to reduced weights of engorged ticks, which results in a low number of eggs laid, or a reduction in hatchability of eggs laid and a low number of larvae hatched. This drastically reduces the tick population in the environment. The disruption in the internal organs in the tick blocks the transmission of the parasite into the host,” Dr. Saimo-Kahwa further explained.
How effective is the anti-tick vaccine against common TBDs?
“The candidate vaccine has already shown 86% efficacy against the R. appendiculatus tick, making it the first anti-tick vaccine locally produced in Uganda and Africa as a whole, which will be used in the control of ticks and tick-borne diseases.”
Concluding remarks
Anaplasmosis poses a serious threat not just to cattle health but also the livestock economy it supports in Uganda. By understanding its cause, clinical signs, diagnosis, and management strategies, stakeholders including farmers, scientists and the government are better positioned to take proactive measures to protect cattle herds and the livestock industry.
The potential to immunize cattle with a vaccine against the lethal tick-borne cattle disease Anaplasmosis and other TBDs presents all stakeholders with an efficacious control and prevention measure that surpasses control levels by chemical acaricides, drugs or other outdated live vaccine approaches that are very expensive.
It is hoped that vaccination using TicVac-U® will help maintain economic levels of tick control and endemic stability, while achieving consistent performance against blue tick populations in Uganda and around endemic regions in Africa, ultimately supporting the agricultural economy and food security in Uganda and other affected regions in Africa. That’s how success would be defined by the multiple actors working behind the scenes to make the vaccine available to farmers as soon as possible.
Currently, the candidate anti-tick vaccine is being produced in a GMP-compliant factory at Alfasan-Uganda Ltd, a Veterinary Drug and Vaccine manufacturer. The candidate vaccine will be taken through rigorous clinical trials to test the immunogenicity and efficacy in the field through many herds of cattle before it is commercialized.
The successful development and production of this anti-tick vaccine using local strains of ticks and manufactured locally, provides a strong rationale to continue further development and use of recombinant proteins as a sustainable, cost-effective and scalable alternative to chemical acaricides, drug treatment and the live infection and treatment technique in the control of ticks and tick-borne diseases or other local diseases decimating the national herd in Uganda.
