Gallsickness, or anaplasmosis, is a tick-borne disease that is common throughout Zimbabwe, and which affects cattle, sheep, and goats. Local cattle breeds are more resistant to the disease than foreign breeds.
Cattle deaths due to the disease occur throughout the year, but in greatest numbers during the summer months when tick infestations are heaviest.
Dipping with chemicals to deter tick infestation is a double-edged sword:
On the one hand it controls tick infestation and disease, while on the other hand it results in a lack of immunity to these diseases. Where natural resistance does not occur, it is important to immunise cattle against tick-borne diseases to prevent it.
What is gallsickness?
Gallsickness in cattle is mainly caused by various anaplasma rickettsia bacteria, including the organism anaplasma marginale, and to a lesser degree, anaplasma centrale, which is less severe than anaplasma marginale.
It is primarily transmitted by ticks where the distribution of mainly three species occurs, namely the African blue tick (Rhipicephalus decoloratus), Asian blue tick (Rhipicephalus microplus), and the red-legged tick (Rhipicephalus evertsi evertsi). It is also caused by some biting flies and mosquitos.
It is important to note that unhygienic conditions, such as using dirty needles and bloody farm equipment like ear notchers that are used to mark animals, can also transmit the disease between animals.
How does anaplasma cause disease?
An infected tick transmits the organism through its saliva when it attaches to the animal. The anaplasma organism enters the animal’s bloodstream and infects the red blood cells, where it starts to multiply. The number of infected red blood cells doubles every 24 to 48 hours.
The incubation period for gallsickness varies but is usually takes between three to four weeks after the tick bite transmission took place. The infected red blood cells become foreign to the animal’s body and are removed by the liver, spleen, and lymph nodes.
The infected red blood cells are broken down and the animal starts to develop anaemia. The animals can lose up to 70% of their red blood cells, but the anaemia normally only becomes noticeable once the animal has lost 40% to 50% of the red blood cells.
The animal’s mucous membranes, namely the gums, eyes, and inner layer of the vulva or preputium (the extended part of the foreskin of the penis that covers the glans penis) turn pale before it turns into the more common and noticeable yellow colour. The yellowing is caused by the broken-down red blood cell pigment or haemoglobin in the liver.
Affected animals show fever, which may disappear during the later stages of the disease, general weakness, loss of appetite and weight loss, rumen stasis (indigestion), and constipation. In severe cases, there is yellowing of gums and eyes, called icterus, which is due to anaemia or loss of red blood cells. In dairy cows, milk production drops severely, and pregnant cows may abort.
Older animals can suffer from the disease for longer and up to 2 weeks. Some animals may show aggressive behaviour, and bulls may become temporarily infertile.
The severity of disease is related to the age of the animal. Animals younger than a year, rarely show any symptoms. Animals between one and two years of age show moderate symptoms, while in older cattle the disease is severe and usually fatal in up to 50% of the cases.
Animals that survive have a convalescence period of several months to regain their former condition.
Gallsickness can be confused with redwater (babesiosis) in the early stages. A veterinarian can help to diagnose the disease correctly. Treatment for gallsickness using oxytetracyclines is usually very successful if diagnosed early. Other supportive treatment like multivitamins can improve the recovery of the sick animals.
Since this is a tick-borne disease, the main control method is consistent dipping.
Consistent communal plunge dipping, knapsack spraying, and spray races can be used to apply chemicals that deter ticks.
Plunge dip tanks are the preferred method for dipping large numbers of especially beef cattle. If constructed and used correctly, these ensure that the animals are completely submerged in the dipping chemical so that it reaches the inside of the ears and under the tail where ticks are found.
Spray races usually consist of a concrete base and metal walls, usually with a tunnel shape and with strategically placed nozzles at different angles to ensure the animal is completely covered with the dip-wash sprays coming through the nozzles. These spray races are used in many tropical and subtropical countries to periodically treat cattle against ticks, biting flies, mosquitos, and other external parasites.
Dipping chemicals should be changed regularly to prevent buildup of tick resistance.
A diagnosis can be made based on the clinical signs, but as the early stages of gallsickness can be confused with redwater, a definitive diagnosis must be made with a microscope on a blood smear.
Carrier animals can be diagnosed with serology tests on the body fluids before they are introduced into the herd. Animals diagnosed with gallsickness can be treated with two recognised active ingredients, Imidocarb (Imizol®) or tetracycline (Reverin 135). Contact a veterinarian to advise which treatments are available in Zimbabwe.
These drugs do not remove the infection and recovered animals will remain lifelong carriers. They may get the disease again and can cause new outbreaks in the herd.
Natural immunity to gallsickness
Bos indicus cattle are more resistant to anaplasma than Bos taurus breeds, but the resistance may vary between breeds.
Calves younger than six months have an innate immunity against anaplasma, despite the cow’s immune status. Cattle raised in endemic areas can develop immunity to the disease before the age when clinical signs become severe.
Control and prevention
The control of gallsickness depends on two strategies, namely tick control by dipping/spraying, and vaccination. Unlike redwater, it is not possible to block anaplasma.
As most parts of Southern Africa are endemic to anaplasma, a stable disease situation must be sustained to prevent large losses.
By allowing calves to be exposed to tick-borne diseases while they are still young and naturally immune, they will increase the antibody levels against these diseases. If this cannot be done, calves younger than six months of age should be vaccinated with the anaplasma vaccine to ensure immunity.
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Du Plessis, C. (n.d.) Understanding Anaplasmosis (gallsickness) https://www.msd-animal-health.co.za/wp-content/uploads/sites/30/2020/05/VV-anaplasmosis.pdf